The need for wireless communication services, such as Cellular Mobile Telephone (CMT), Personal Communication Services (PCS) and the like, typically requires the operators of such systems to serve an ever increasing number of users. As a result, certain types of multichannel broadband Base Transceiver Systems (BTSs) have been developed which are intended to service a relatively large number of active mobile stations in each cell. Such broadband BTS equipment can service, for example, ninety-six simultaneously active mobile stations, at a cost of less than $2000 to $4000 per channel.
While this equipment is cost effective to deploy when a relatively large number of active mobile stations is expected in each cell, it is not particularly cost effective in most other situations. For example, during an initial system build out phase, a service provider does not actually need to use large numbers of radio channels. As a result, the investment in broadband multichannel radio equipment may not be justified until such time as the number of subscribers increases to a point where the channels are busy most of the time.
There are certain techniques for expanding the service area of a cell site. For example, the HPT Cell Site Expander product manufactured by 3dbm Inc., of Camarillo, Calif., consists of a base station translator which samples downlink signal traffic and translates it to a selected offset frequency. The offset carrier is transmitted to an expansion cell site via directional antennas. At the expansion cell site, the carrier is translated back to the original cellular channel and transmitted throughout the expansion cell site coverage area such as via an omni-directional antenna. In the uplink direction, a cellular signal received by the expansion cell site from a mobile unit is translated and then transmitted back to the base station translator, which in turn translates the signal back to its original carrier frequency.
However, there are still other complex engineering considerations to maximize cell site efficiency. In particular, the system operator must also split up the allocated frequency channels among the cells, so that units operating in adjacent cells do not interfere with one another. In such a scenario, only a fixed number of transmit and receive operating frequencies are thus made available to service the mobile units in each cell. Movement of a mobile unit across a cell boundary must therefore be detected, so that the mobile can be reassigned a new pair of frequencies on which to operate in the new cell. This process, known as hand-off, must occur quickly, so that no interruption of a call in progress can be perceived. Unfortunately, at certain cell densities, the time to process a hand-off may become a significant factor in the ability of such systems to consistently provide reliable telecommunication service.
There are at least two factors which determine the speed at which a hand-off must occur, including (1) the rate at which the mobile unit passes through the cells, and (2) the extent to which non-uniformities in the radiated electromagnetic field in the cell affect the ability to accurately detect the signal from the mobile unit. Both of these factors depend upon the time required to accurately determine the relative location of the mobile unit. With respect to the speed of movement through the cells, in certain proposed PCS systems, the cells may be as small as five hundred (500) feet in radius. Thus, a mobile unit traveling only a few feet may require the handing off of the unit from one base station to a second and perhaps to even a third base station.
With respect to the second factor, because electromagnetic fields are usually non-uniform, a measurement of signal strength is typically made a number of times and then averaged. The time required to perform this measurement becomes longer as the susceptibility of the electromagnetic field to fading effects increases, such as may occur in an urban environment.
Diversity combining techniques can be used to compensate for such fading by generating a number of signal transmission paths, or diversity branches, each of which carry the same information signal, but which have uncorrelated multipath fadings. The diversity branches then are combined in some way to resolve the actually transmitted signal. It would be desirable to reduce the complexity of the operations required in detecting the position of a mobile unit, by taking advantage of diversity combining techniques in as efficient a manner as possible, with a minimum number of base station antennas and associated receiver processing and control equipment.
This can, however, be especially difficult in the case where the base station makes use of a wideband receiver which provides signals from many different remote units at the same time, because the multipath fadings must be compensated for each radio channel.
Furthermore, while a remote translator may be used to extend the radius of a cell, known designs do not suggest how to implement diversity selection among signals received from multiple mobile stations located in multiple expansion cells simultaneously. This is especially the case in a Time Division Multiple Access (TDMA) system, where the receive signals must be properly timed.